This invention relates to a drive circuit for the operation of electronic power switches and power supplies employing switching regulators and, more particularly, to the driving of such a switch wherein the switch comprises a MOS field effect transistor (FET).
DMOS power MOSFETs are relatively new electronic components which have become commercially available from a number of manufacturers in the United States including Motorola, Unitrode, and International Rectifier Corporation. Such MOSFETs differ from PNP bi-polar transistors in that a larger voltage between gate and source is required to initiate conduction of current than the corresponding voltage between base and emitter of the bi-polar transistor. Of particular interest is the design of switching regulators with respect to the increased speed of turn-off, or termination of current conduction, in the MOSFET. The MOSFET can be switched off in approximately 0.1 microseconds or less as compared to 2 microseconds for a bipolar transistor of comparable power rating.
One form of switching regulator is known as a buck switching regulator. In such a regulator, the power switch transistor floats at the line voltage of the regulator. Sensing circuitry in the regulator is at ground potential. The sensing circuitry senses the output voltage of the regulator and provides a pulse-width modulated control signal wherein the duty factor determines the average value of the current to be conducted by the power switch to the regulator output.
A problem arises in the coupling of the control signal to the switch transistor due to the fact that the control signal must be coupled from the sensing circuit, at ground potential, to the switch which floats at the line potential. Heretofore, PNP bi-polar transistors have often been used as part of the power switching element. Transformers and optical couplers have been often used for coupling the control signal to the power switch transistor. Optical couplers are disadvantageous in that their operation is limited to lower power levels requiring additional post amplification. Also, the frequency response of the optical couplers is significantly lower than that of the power MOSFETs so that a major advantage in the use of the MOSFET is depressed.
The aforementioned problems associated with coupling a control signal from the sensing circuit at ground potential, to the switch, which floats at line potential have led others to use transformers or photocouplers to translate the input drive signal from ground reference to line reference. The circuit shown in Application Note 950 from the 1982-83 Hexfet Databook (published by International Rectifier) is a good example of the prior art approach. A saturable core coupling transformer is used in this prior art circuit. The instant invention solves the ground translation problem without using transformers or opto-couplers and thus eliminates the drawbacks associated with these components. The transformerless coupling circuit of the instant invention offers better high frequency noise immunity than the transformer type circuits of the prior art.
Transformers also introduce limitations in the operating characteristics of the regulator in that they cannot effectively couple the control signal in the cases of very low and very high duty cycles. A further disadvantage of the transformers found in the production line is that transformers from different manufacturers tend to differ in their characteristics due to limitations in the comprehensiveness of specifications. Thus, the resulting characteristics of the regulator differ depending on the source of the transformer and dependence on unspecified transformer parameters. Comprehensive specifications which would reduce variations between transformers would cause a significant increase in cost.